1. Field of the Invention
The present invention relates to an efficient process for producing a polyimide soluble in specific solvents which is excellent in storage stability, handling property and processability.
2. Description of the Prior Art
As conventionally known in the arts, solvent-soluble polyimides have been produced by a high-temperature solution polymerization method or a chemical imidation method. In the high-temperature solution polymerization method, a polyamic acid having a sufficiently high molecular weight is first produced, and then heated in the presence of a necessary and sufficient amount of a water-azeotropic agent such as aromatic hydrocarbons while removing generated water out of the reaction system, using an apparatus capable of removing only water out of the reaction system but recycling the water-azeotropic agent into the reaction system (Japanese Patent Application Laid-Open No. 2003-119284). In the chemical imidation method, a polyamic acid having a sufficiently high molecular weight is first produced, and then imidated at an ordinary temperature or under heating using an excessively large amount of a chemical imidation agent such as acetic anhydride (Japanese Patent Application Laid-Open No. 2002-80596).
However, the high-temperature solution polymerization method has various problems such as prolonged time required for producing polyamic acid having a sufficiently high molecular weight, deficient increase in molecular weight and imidation rate due to failure to rapidly remove water out of the reaction system, necessity of using the water-azeotropic agent and special apparatus, deterioration in solubility of the polyimide due to adverse influence of the water-azeotropic agent, and high costs in productive facilities and production. The chemical imidation method has problems such as prolonged time required for producing polyamic acid having a sufficiently high molecular weight, necessity of purification process for removing an excessively large amount of the chemical imidation agent such as acetic anhydride, and high costs in productive facilities and production.